Spray drying equipment and method



1951 D. D. PEEBLES ET AL 2,575,119

SPRAY DRYING EQUIPMENT AND METHOD Filed Aug. 22, 1949 2 $HEETSSHEET lINVENTORS Da via 0. Peeb/cs Peg/na/a E eaae BY 7 I ATTORNEYS PatentedNov. 13, 1951 2,575,119 SPRAY p fmc: EQUIPMENT AND METHOD DavidDJeebles, Hillsborough, Calif., and Regb nald E; Meade, Appleton, Wis.,assignors to Western Condensing Company, San Francisco, Calif., acorporation of California Application August 22, 1949, Serial No.111,638

3 Claims. (01. 159-41b' This invention relates generally to spray dryingequipment and methods such as are used to convert various liquids andslurries to powdered Conventional types of spray drying equipmentutilize a desiccating chamber equipped with a suitable atomizing device.A source of hot' air or other drying gas is continuously introduced intothe desiccating chamber, whereby the atomized particles are suspended inswirling currents of the drying gas, thereby removing suflicient waterto form a divided or powdered material. In some instances the powder iscentrifugally separated out in the lower part of the desiccating chamber(see Gray and Jensen Patent No. 1,107,784) and the spent drying gas isremoved through a separate exhaust conduit. In other instances bothexhaust gases and the powder are removed together, and the powderseparated externally of the desiccating chamber. Because of theircapaciy and ability to handle a wide range of liquid material andslurries, centrifugal atomizing devices have found increasing favor forsuch equipment. The centrifugal atomizing device may consist of a discmounted on a vertical shaft, and driven at a relatively high speed ofrotation. The liquid to be atomized is delivered to theupper face ofthis disc, and the periphery of the disc is provided with a plurality ofcircumferentially spaced teeth from which atomized particles aredischarged.

Stationary atomizing nozzles may be used in place of centrifugaldevices, particularly where the material is such that the problem ofatomization is not difficult, or where the capacity requirements aresuch that a stationary atomizer is feasible. The hot air or other drygas is generally introduced into the desiccating chamber tangentially,to promote the desired swirling movement, and as the hot air enters thechamber it is immediately guided in a curved path by engaging adjacentcurved walls offlthe chamber.

Various difiiculties have been experienced with conventional desiccatorsand methods of the above type. For example it frequently happens thatthe atomized particles of material vary considerably as to particlesize, and this may result in an unsatisfactory final product. Should thedrier be operated in such a manner as to leave a desired moisturecontent in the larger size particles, then the smaller particles tend todry to anhydrous condition, thus resulting in a final product in whichthe moisture content is not unlformly distributed. If it is attempted tooperate dried to a desired free moisture content, then the largerparticles tend to have too much excess free moisture, thus likewiseresulting in an unsatisfactory final product. These difiiculties areparticularly noticeable when handling material which is to be dried toleave a desired percentage of water of hydration in the final material,to render the same nonhygroscopic. Because of these difficulties it hasbecome customary practice to operate a spray drier in such a fashionthat the majority of the atomized particles are of relatively smallsize. In the case of centrifugal atomizers this is accomplished byoperating the atomizing device at relatively high speeds, while in thecase of stationary atomizers it is accomplished by utilizing relativelyhigh atomizing pressures. Atomizing to the extent of causing the bulk oftheatomized particles to be relatively finely divided has thedisadvantage that the final product may have an objectionably smallparticle size, and in addition it is well known that excessive atomizerspeeds or pressures, are objectionable.

In addition to the above it has been noted that conventional spraydriers equipped with either centrifugal or stationary atomizing devicesdo not have the capacity theoretically attainable. This has beenattributed to lack of proper distribution of the suspended atomizedparticles in the desiccating chamber, whereby there is considerable lackof uniformity with respect to the number of particles present at a:given instant in a unit of space, at various points within thdesiccating chamber.

In general it is an object of the present invention to provide a newdesiccating apparatus and method which avoids the aforementioneddifficulties and limitations of prior conventional spray dryingequipments and methods.

Another object of the invention is to provide a new desiccatingapparatus and method which is relatively flexible with respect to thesizeof the atomized particles which may be satisfactorily dried, andwhich will facilitate production of a dried product to a desiredmoisture content, with a minimum variation in moisture content of thevarious particles. I

Another object of the invention is to rovide a new desiccating apparatusand'metliodfcharacterized particularly in that it has relatively highcapacity and will enable the use of relatively high inlet gastemperatures without causing burning or heat injury to the materialbeing dried.

the drier so that the smaller particles will be Another object of theinvention is to provide a desiccating apparatus and method which servesto maintain free flight of the atomized material within the desiccatingchamber, without permitting direct impact of material discharged at highvelocity in a horizontal plane from the atomizin device on heated wallsof the chamber.

Another object of the invention is to provide a desiccating apparatusand method making possible optimum free flight distribution of theatomized particles in the drying gas, thereby promoting high capacityand uniformity with respect to the spray dried product.

Additional objects of the invention will appear from the followingdescription in which the preferred embodiments have been set forth indetail in conjunction with the accompanying drawmg.

Referring to the drawing:

Figure 1 is a side elevational view, diagrammatically illustrating a.desiccating apparatus incorporating the present invention.

Figure 2 is a plan view of the apparatus shown in Figure 1.

Figure 3 is a side elevational view of another embodiment of theinvention.

Figure 4 is a plan view of the apparatus shown in Figure 3.

That form of the invention illustrated Figures 1 and 2 of the drawingconsists of a desiccating chamber III which is formed symmetrical aboutthe vertical axis H, and which may be annular in transversecross-section. That portion Hla of the desiccating chamber is conicalshaped as illustrated, to facilitate separating powder from the dryinggas. Conduit I2 connects with the lower end of the chamber, and enablesremoval of the dried powder together with a small amount of conveyinggas. Exhaust conduit l3 has an open inlet end I located concentric withthe axis H, and at a level considerably above the point of connection ofconduit l2. Conduit I3 is adapted for connection with the inlet side ofa blower or like draft means.

The upper portion Nb of the desiccating chamber is substantiallycylindrical in form, and receives hot drying gas from the ducts l6, l1,l8 and I9 (Figure 2). As will be presently explained these ducts arearranged in a particular manner. Disposed concentric with the axis II,and likewise in the upper part of the desiccating chamber, there is anatomizing device 2|, which may be of the centrifugal type. The detailsof this device have not been illustrated, because it may be madeaccording to any one of a number of prior art atomizing units. Goodresults have been secured by utilizing the improved centrifugalatomizing device disclosed and claimed in copending application SerialNo. 626,219, filed November 2, 1945/ and now matured into Patent Number2,473,035.

In addition to the hot air ducts'li, l1, l8 and I9 it is desirable toprovide for the introduction of additional hot drying gas into a zoneimmediately surrounding the atomizing device 2|. Thus a hot air inletopening 22 is illustrated in Figure 1, which is circular in contour andwhich is concentric with the axis This opening connects.

with the chamber 23, which in jturn'con'nects with the hot air supplyconduit 2d.

With respect to the positioning of the ducts l6, ll, I8 and 15, it willbe noted from Figure 2 that they are not tangential with respect to theadjacent peripheral wall of the desiccating chamber. Each duct isdirected generally toward the region of the discharge end of the nextadjacent duct. As illustrated in Figure 1, taken together with Figure 2,the axes of these ducts are disposed in a common plane, and this planeis likewise the general plane in which the atomizing device 2! islocated. It will be evident that with the arrangement just described theprojected axes 26, 21, 28 and 29 form substantially a square asillustrated in Figure 2. Likewise the angle of each axis, with respectto an axis which would be truly tangential to the adjacent peripheralwalls of the desiccating chamber (angle a, Figure 2) is in this instanceabout 45. The discharge from each conduit therefore does not follow andis not guided by the adjacent peripheral chamber walls, but follows theline of the projected axes 26, 21, 28 and 29, or in other words itfollows a generally linear path extending between the discharge ends oftwo adjacent conduits.

Operation of the apparatus described above, and the'carrying out of thepresent method, can be described as follows: Assuming that a liquidmaterial containing solids in suspension, such as milk concentrated tofrom 40 to 60% solids is being supplied to the atomizing device 2|, thismaterial is discharged outwardly from the atomizer at relatively highvelocity, and the initial direction of flight is horizontal and towardthe surrounding peripheral walls. Conduits l6, ll, l8 and i9, togetherwith conduit 24, are connected to a source of drying gas, such as ablower and heater serving to deliver hot air at a suitable temperature,and with substantially equal distribution through the conduits IS, IT,I8 and I9. Due to the cross-flow action of hot drying air beingdelivered downwardly from the opening 22 about the atomizing device,some of the atomized particles, are immediately directed downwardly,while other particles continue a substantially horizontal path ofmovement. The latter particles continue outwardly until acted upon bythe hot air jetting from the ducts l6, l1, l8 and 19. These hot aircurrents are of suflicient velocity to deflect such particles and carrythem along in suspension, thus preventing the particles from continuingtheir movement until contacting the surrounding peripheral walls. Afterbeing acted upon by the hot air blasts from the ducts l6. l1, I8 and i9,atomized particles progress downwardly into the main part of thedesiccating chamber, thus partaking of a sustained free flight the sameas the atomized particles which were initially deflected downwardly.There is a general swirling or cyclonic movement of gas and suspendedparticles in the desiccating chamher, with the result that there is acentrifugal separating action, which causes the dried particles toprogress downwardly about the conduit [3, to be finally removed throughconduit I2. Spent gas is continuously exhausted through the conduit B.When viewed from the side as in Figure l, the path of movement ofatomized particles leaving the atomizing device 2|, is either directedoutwardly until acted upon by hot air discharging from ducts l 6, l1, l8and I9, and then downwardly in a spiral path in the main part of thedesiccating chamber, or generally outwardly and downwardly from theatomizing device, with spiral movement until the particle is finallycollected in the lower part of the chamber.

The desiccating method and apparatus described above has many novelcharacteristics. Assumingthat the atomizer produces particles which varyas to size, such particles can be dried more uniformly, to produce aflnal product having a free moisture content which is relativelyuniformly distributed through both the coarse and finer particles. Thecoarse or larger atomizer particles tend to be discharged directlyoutwardly from the atomizer 2|, to be acted upon and entrained by thejets of hot air discharging from the hot air inlets IE to l9 inclusive,and to be thereafter caused to swirl about the axis of the chamber andto progress downwardly into the region extending generally below theatomizer and surrounding the central axis. The finer atomized particlestend to be deflected downwardly by hot air entering through the inletpassage 22, and to likewise be suspended in swirling currents in thedesiccating chamber, without first being acted upon by the freedischarge issuing from the conduits |6l9. The larger or coarserparticles are therefor caused to traverse a longer path of travel tothereby reduce the moisture content to a value consistent with thereduction in free moisture content of the finer particles. Thus asuperior powdered product can be produced consist g of both coarse andfine particles, with a rel tively uniform distribution of residualmoisture content.

The feature just described can be utilizedto advantage in the drying ofmaterials where it is desired to provide a specified amount of residualfree moisture content in the final material. For example the drying oflacteal material containing substantial amounts of lactose (e. g. whey),it desirable to producea" final product by a spray drying operationwhich has a sufficient amount of moisture to provide for water ofhydration of the lactose present, together with an additional amo unt offree moisture. In the past it has been customary to provide for asubstantial free moisture content in the final product, such as say from8 to 20% in order to insure ample water of hydration in both the coarseand finer particles, and this practice has necessitated the use ofsecondary drying after the initial primary spray drying operation. Withthe present apparatus and method the free moisture content, over andabove the water of hydration required, can be kept to a relatively lowvalue, such as from 2 to 5%, with a uniform distribution of moisture inboth the fine and coarse particles, thus obviating secondary drying.

As previously stated it has been customary practice in the past tooperate atomizers for spray driers in such a manner as to atomize thebulk of the material to relatively small particle size. This hasrequired use of relatively high rotative speeds for centrifugalatomizers, or relatively high fluid pressures for the stationary type ofatomizers, both of which are objectionable. With the present inventionit is possible to depart from such conventional prior practice. Thus itis possible to operate the atomizing devices at lower speeds orpressures, in order to produce a larger percentage of coarser particles,and such coarser particles can be adequately dried to a desired moisturecontent by the use of our apparatus and method. In prac-, tice thisfeature makes it possible to reduce the the emphasis upon production ofrelatively small sized atomized particles, and the bulk of the atomizedparticles produced by the atomizing device may be relatively coarse. Inthe production of many commercial products this feature is highlydesirable, because it enables the production of final products of acoarser grain. In addition operation of the apparatus in this mannerserves to effectively reduce power consumption and maintenance costs ofthe atomizing devices.

cross-sectional contour.

' Previous reference has been made to the fact that in conventionalspray drying equipment the incoming hot gas is guided by the adjacentwalls of the desiccating chamber. With the present invention theincoming hot gas is no longer directly guided by adjacent walls of thechamber,'and fiie of'tlie' walls constituting the upper chamber pa temain at a relatively low temperature. This *because incoming gases,before contacting these walls, have been cooled by evaporation ofmoisture from the atomized material. Because of this feature anymaterial which comes into direct contact with the walls of the chamberportion 10b, are not burned or injured by excessive heating.

The jets or streams of hot gas discharging from the conduits Hi to I9afford a greater degree of turbulence within the main part of the dryingchamber. This is because the incoming streams of gas initially havesubstantially linear flow, and thereafter swirl generally about the axisof the chamber, and within the main portion of the drying zone. Greaterturbulence tends to accelerate evolution of moisture from the atomizedparticles, thus enabling a corresponding increase in the capacity of theequipment.

It will be evident that the invention is not limited to the use of fourhot an inlet ducts. Thus it is possible for example to use five or sixduets, with the distribution such that they all discharge into thedesiccating chamber on axes located in a common plane with the generalplane of the atomizing device, and with each axis being directedgenerally toward the discharge opening of an adjacent duct. It will beevident that the axis 21 of each duct need not be directed preciselytoward the outlet of the adjacent duct. The general direction issufficient provided the air is discharged from each duct with sufficientvelocity and such direction as to sweep directly over a generally linearpath to intermingle with air discharged from the next duct. Thus thetheoretical discharge axes 26, 21, 28, 29 may each bedirected tointersect the adjacent axis at a point somewhat beyond the outlet end ofthe latter.

In that embodiment of the invention illustrated in Figures 3 and 4, thedesiccating chamber 3| is likewise formed symmetrical about the verticalaxis 32, and is annular in transverse The lower portion 3Ia of thechamber is conical shaped to aid in centrifugal separation of driedpowder. The dried powder in thisinstance is removed through the loweroutlet conduit 33. The upper portion 3Ib of the desiccating chamber iscylindrical, and connects with the hot air ducts 36, 31, 38 and 39(Figure 4). These ducts all connect with a common hot air supply conduit4|, which in turn connects to the discharge side of a blower, throughthe air heater 42. Spent drying air is removed through the exhaustconduit 43, which connects with the cyclone separator 44. The upper openend 46 of the conduit 43 is located concentric with the xis 32, andintermediate the upper and lower ends of the desiccating chamber. Thecentrifugal atomizing device 41 is located above the openend 46 of theexhaust conduit, and is in the same general plane as the ducts 36, 31,38 and 39. A hot air inlet opening 48 is located adjacent and above theatomizing device 41, and it receives hot air from the chamber 49, andconduit 5|. This conduit can connect with the outlet side of the sameheater 42. An additional conduit 52 can be provided for introducingsupplemental drying air, and it is shown communicating tangentially withthe chamber at a level below the end 46 of the exhaust conduit 43.

The apparatus of Figures 3 and 4 possesses the same features as thepreviously described apparatus. The atomized particles delivered by thedevice 41 will be prevented from directly impacting the surroundingperipheral walls of the desiccating chamber, and the atomized particleswill take a free flight in which they swirl about the axis 32, andgradually progress downwardly until separated out in the lower end oithe chamber. The spent drying air removed through conduit 43 may carrywith it a certain amount of classified dried material, and this isremoved in the cyclone separator 44.

We claim:

1. In a spray drying method making use of a vertical drying zone, thesteps of introducing atomized particles of material to be dried into theupper portion of said zone adjacent the vertical axis thereof andprojecting said particles generally horizontally from said axis,continuously introducing hot drying gas into the upper portion of thezone to maintain a diving atmosphere within the zone for drying theatomized particles, thehot drying gas being introduced as freedischarging jets of gas distributed about the horizontal region in whichthe atomized particles are introduced, said jets being on substantiallylinear axes which successively intersect and which are in a conmionplane at right angles to the said vertical axis of the zone andsubstantially coincident with the horizontal plane in which the atomizedparticles are introduced and projected, said jets being localizedwhereby the direct discharge of the same into the zone is confined tothe proximity of the upper and lower sides of said plane.

2. In a spray drying method making use of a vertical drying zone, thesteps of introducing atomized particles of material to be dried into theupper portion of said zone adjacent the vertical axis thereof andprojecting said particles generally horizontally from said axis,continuously introducing hot drying gas into the upper portion of thezone to maintain a drying atmosphere within the zone for drying theatomized particles, the hot drying gas being introduced as freedischarging jets of gas distributed about the horizontal region in whichthe atomized particles are introduced, said jets being on substantiallylinear axes which successively intersect and which are in a common planeat right angles to the said vertical axis of the zone and substantiallycoincident with the horizontal plane in which the atomized particles areintroduced and projected,

said jets being localized whereby the direct discharge of the same intothe zone is confined as to the proximity of the upper and lower sides ofsai plane, introducing additional hot drying gas into a regionsurrounding the region into which the atomized particles are introducedand in a downward direction, to thereby deflect the smaller I sizedparticles downwardly into the main portion of the chamber, whilepermitting coarser particles to be discharged laterally into said jetsof hot drying gas.

3. In a spray drier, a desiccating chamber formed symmetrical about avertical axis, the lower portion of said chamber being provided withmeans for removal of powder and drying gas. a centrifugal atomizingdevice disposed in the upper portion of the chamber concentric with saidaxis and arranged to project particles outwardly in a generallyhorizontal plane, a plurality of ducts adapted to receive hot primarydrying gas and communicating with the upper partof the chamber throughthe peripheral wall of the same, said ducts being spacedcircumferentially at regularly spaced intervals and being disposed todischarge on axes which are substantially in a horizontal planecoincident with the plane in which said atomizing device projectparticles, the axis of each duct being directed chordally of saidchamber and intersecting the axis of a jet issuing from an adjacent ductand all of said ducts discharging in-a zone which'is confined to theproximity of said common plane, and means .a

for introducing supplemental gas into said chamber in a region generallysurrounding the centriiugal atomizing device and downwardly with respectto said axis, and serving to receive and entrain fine particles ofmaterial discharged laterally of the device and to cause such materialto progress into the lower portion of the desiccating chamber.

DAVID D. PEEBLES.

REGINALD E. MEADE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,107,784 Gray Aug. 18, 19142,081,909 Bowen June 1, 1937 2,151,079 Bowen -i- Mar. 21, 1939 2,184,314Peebles Dec. 26, 1939 2,473,035 Meade et al June 14, 1949 FOREIGNPATENTS Number Country Date 317,166 Great Britain Aug. 15, 1929

